The trouble is that the defaults tend not to be the best fonts that are available, and very few users change them. I have changed them myself, but I don’t know of anyone else that has.
For myself, I wish that people would leave Arial, Verdana, Helvetica Neue, Helvetica, &c. out of their sans-serif stack, having only their one preferred font and sans-serif, or better still sans-serif alone; but as a developer I understand exactly why they do it all.
Unfortunately, I'm one of those developers :( My font stack is:
font-family: system-ui, Helvetica, sans-serif;
for prose and
font-family: ui-monospaced, Menlo, monospace;
for monospaced text. The first being the user's preferred font, the second as a good (IMO?) default that I impose on them, and the third as a full fallback. I'm conflicted on whether this is the right balance between user choice and handling browsers that support nothing.
For myself, I wish that people would leave Arial, Verdana, Helvetica Neue, Helvetica, &c. out of their sans-serif stack, having only their one preferred font and sans-serif, or better still sans-serif alone; but as a developer I understand exactly why they do it all.